1887

Abstract

Impact structures produced by meteorite or cometary impacts are generally circular features, which are accompanied by a range of diagnostic shock phenomena. Many other processes can also produce circular structures that resemble impact structures. We report on two case studies from Zimbabwe, where we investigated two circular possible impact structures, in similar host rocks, using N-S traverses with a hand-held proton precession magnetometer. The West Save structure, in eastern Zimbabwe is a ~600m diameter structure, developed in Triassic sandstones of the Karoo Supergroup. The ~220m diameter Sinamwenda structure, in western Zimbabwe, near Lake Kariba, is also developed in similar Triassic Karoo sandstones. The West Save structure contains basalt and gabbroic intrusions, and is formed by differential erosion of a more indurated sandstone rim around a volcanic pipe. At West Save a strong asymmetric jagged dipole magnetic anomaly, ranging from 30140 to 31080 nT, is interpreted as a signal produced by a vertical cylinder of intrusive rock in a volcanic vent. The Sinamwenda structure, by contrast, has no trace of volcanic or intrusive rocks, and it shows not the slightest magnetic anomaly, with a constant total magnetic field value of 31000 nT, with a variation of the magnetic intensity of only 2 nT. An overturned rim, multiply striated joint surfaces, microbrecciation, and planar deformation lamellae in quartz indicate an impact origin for the Sinamwenda structure. The lack of a magnetic anomaly rules out alternative interpretations such as kimberlitic intrusions. We conclude that ground magnetic study of circular structures can easily distinguish between structures produced by impact processes (absence of magnetic anomaly), and structures produced by volcanic intrusions (dipole magnetic anomaly), if the magnetic susceptibility of the surrounding rocks is minimal.

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/content/papers/10.3997/2214-4609-pdb.241.master_paper1
2009-09-16
2024-03-29
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.241.master_paper1
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